1. Field of the Invention
The present invention relates generally to the analysis of gases by the use of infrared radiation and, more particularly, to a non-dispersive infrared correlation spectrometer having a high degree of sensitivity and specificity.
2. Description of the Prior Art
It is well known in the art to employ the principle of absorption of infrared radiation in a gas mixture as the basis for qualitative analysis of the mixture and, in particular, for the quantitative determination of the presence of one or more specific constituents in that mixture. Such instruments possess great practical potential in monitoring industrial effluent and process streams, exhaust analysis and other important environmental and ecological considerations.
In one type of analyzer, the infrared energy absorbed from a reference beam is compared with that absorbed from an analysis beam sensitized with the "gas of interest" (i.e., the gas sought to be determined in the sample) in a manner which renders the difference between the energy absorbed from each of the two beams dependent only upon the amount of the specific gas of interest present in the sample. Thus, the reference beam traverses a cell filled with a gas which does not absorb infrared radiation and a sample cell which may or may not contain the gas of interest and proceeds finally to a detector which produces a signal in response to the amount of infrared radiation received. The signal produced by the absorption from this beam is then compared with the signal produced by the absorption from a second or analysis beam which traverses a sensitizing cell containing the gas of interest and the sample cell and impinges on the detector. Both of these beams may be passed through additional filter cells in which an interfering gas, one which has infrared absorption spectrum overlapping the infrared absorption spectrum of the gas of interest, may be placed to filter out the effect of that particular component if such exists in the sample. The detector used may be of any type suitable for the absorption of infrared in the spectral range required.
One great problem associated with all of these prior art devices is concerned with the inability to develop an analyzing technique which does away with the detection problems caused by interfering gases in the sample and, at the same time, provides a fast, accurate quantitative determination of the gas of interest which is independent of fluctuations in the source output and detector absorption. Another problem concerned with quantitative accuracy in prior art devices is in the detection of small quantities of a gas of interest. A small difference in two relatively intense beams must be measured and even a small error in this situation can produce a large error in the quantitative measurement.